A highly divergent member of the class I MHC gene family, M1, represents a direct descendant of a primordial class I gene, which antedates speciation. Using a series of recombinant and congenic strains of mice, this gene has been mapped to a region of chromosome 17 telomeric to Qa. In conjunction with studies of other divergent class I genes, such as those encoding HMT, M1 has been used to define a new MHC subregion, M. Despite the extensive divergence of M1 from the rest of the H-2 class I family, M1 has open reading frames and legitimate splice sites in all exons. M1 appears to contain a functional promoter since introduction of a 3.4 kb genomic DNA fragment containing M1 into mouse L cells results in transcription of the gene. Nevertheless, M1 is not detectably transcribed in a variety of cell lines or adult somatic tissues and no M1 antigen has been identified. Expression of M1 is controlled by a strong silencer element located within a 16 kb genomic fragment containing the M1 gene.